So far, many researches and developments for the usage of lithium metal, which can actualize a high energy density with a high voltage, for a negative electrode of a non-aqueous electrolyte secondary battery have been conducted. However, a battery excellent in cycle life and safety is not easily obtained. Thus, presently, lithium ion secondary batteries in which a graphite type carbon material capable of reversibly absorbing and desorbing lithium is used for the negative electrode are available in practical use.
However, the theoretical capacity of graphite is 372 mAh/g. The batteries available in practical use already utilize a capacity of about 350 mAh/g. Therefore, the usage of carbon material does not meet the expectation of realizing a non-aqueous electrolyte secondary battery with sufficient capacity as a future energy source of high-performance mobile devices. For achieving a further higher capacity, a negative electrode material having a theoretical capacity higher than that of graphite is necessary.
Thus, alloy materials including silicon or tin are gaining attention. Silicon and tin are capable of electrochemically absorbing and desorbing lithium ion. Silicon and tin also have a much higher capacities compared with that of graphite. For example, theoretical discharge capacity of silicon is 4199 mAh/g, about 11 times higher than that of graphite.
However, alloy materials form alloys such as a lithium-silicon alloy and a lithium-tin alloy when absorbing lithium. Therefore, crystal structure of the alloy materials changes along with charge and discharge. The changes in crystal structure involve a great deal of change in volume. For example, the volume of silicon theoretically expands to 4.1 times more, when silicon absorbs lithium to the maximum. Thus, an active material comprising alloy material easily separates from the current collector. As a result, electric conductivity in the negative electrode is lost, to greatly deteriorate high-rate discharge characteristic and charge and discharge cycle characteristic. On the other hand, when graphite absorbs lithium, lithium is intercalated between graphite layers. Such intercalation reaction only expands the volume of graphite to 1.1 times.
Japanese Laid-Open Patent Publication No. 2002-83594 (D1) proposed a negative electrode comprising an active material portion composed of amorphous silicon on a current collector with roughened surface to cope with the expansion of the negative electrode material. By roughening the surface of current collector, the bond between the active material portion and the current collector becomes stronger. Additionally, the active material portion cracks along the roughened surface to form columnar particles. Such columnar particles can decentralize the expansion stress. However, on the interface between the active material portion that expands and the current collector, cracks and destruction easily occur due to the difference in stress. Therefore, some means for decreasing the interface stress are necessary. Thus, forming a buffer on the interface is proposed by heating the negative electrode to disperse Cu forming the current collector to the active material portion.
However, in such heating, controlling the Cu diffusion is extremely difficult. When Cu is diffused in excess, an inactive phase which does not react with Li increases in the active material portion, to decrease battery capacity. On the other hand, when the diffusion of Cu is too small, the active material portion cannot endure the expansion stress. Thus, cracks and destruction occur on an interface between the active material portion and the current collector.
Japanese Laid-Open Patent Publication No. 2003-217576 (D2) proposes to produce CuO by oxidizing the current collector surface, and then form a film of active material portion comprising Si on the surface. CuO suppresses the excessive diffusion of Cu. Japanese Laid-Open Patent Publication No. 2003-308832 (D3) proposes to form Si films on both sides of the current collector simultaneously by sputtering. This proposal intends to make thermal hysteresis even on front and reverse sides of the current collector to suppress the diffusion. Further, in Japanese Laid-Open Patent Publication No. 2002-373644 (D4), excessive diffusion of the constituent element of current collector is prevented by forming an intermediate layer comprising Mo or W on the current collector surface.
In the proposals of D2 to D4, control of the diffusion of elements becomes easier compared with the heating process proposed in D1. However, the fundamental problems are not resolved, that is, the heating process which cannot be controlled easily is necessary. Further, the proposal of D2 has the demerit of higher resistance, since CuO is formed on the interface. In the proposal of D3, the sputtering involves a large-scale process, to drastically increase the process cost. In the proposal of D4, the heating temperature to bond the active material and the intermediate layer is high. Thus, the active material of micro crystalline or amorphous state is crystallized by the heating process, to deteriorate the electrode performance.